The endogenous opioid receptor-like 1 (ORL1) ligand, orphanin FQ (OFQ), a heptadecapeptide structurally resembling dynorphin A, has recently been identified in rat brain. However, the physiological role of OFQ in gastrointestinal (GI) tract is unknown. Our preliminary studies have shown that OFQ preferentially stimulates muscular contraction in colon without affecting the motility of upper GI tract in rats. We have demonstrated that OFQ accelerates colonic transit and evokes propulsive colonic movements. In contrast, dynorphin A delays colonic transit and produces non-propulsive movements. Based on our preliminary studies, we hypothesized that OFQ acts on colonic myenteric plexus and causes contractions by inhibiting an inhibitory ATP neural pathway of the colonic myenteric plexus. Inhibition of this inhibitory neural pathway will result in colonic muscle contraction and accelerates colonic transit. In contrast, Dynorphin A inhibits ACh, SP, NO, VIP and PACAP release and stimulates random non-coordinated muscle contraction resulting in delayed colonic transit. To test this hypothesis, we plan to perform immunohistochemistry to demonstrate that OFQ is present in abundance in the colonic myenteric plexus and demonstrate that OFQ receptors are expressed in the colonic myenteric plexus by in situ hybridization. We will then characterize the neural pathways responsible for the stimulatory action of OFQ on colonic motility and investigate the cellular mechanism mediating the inhibition of an inhibitory pathway by electrophysiological studies. Finally, the inhibitory neurotransmitter inhibited by OFQ will be identified by pharmacological studies as well as immunocytochemistry. In addition, we also plan to compare and contrast the mechanisms of action of OFQ versus dynorphin A on colonic motility. We will demonstrate that OFQ, but not dynorphin A, causes propulsive colonic movements and accelerates colonic transit in vivo. Dynorphin A, but not OFQ, inhibits ACh/SP/NO/VIP/PACAP pathway which mediates the peristaltic reflex. In addition, we will perform combined procedure of in situ hybridization and (immuno)histochemistry to demonstrate that OFQ receptors are present on ATP containing neurons whereas kappa receptors are present on ACh-, SP-, NOS-, VIP-, or PACAP-containing neurons. These studies will shed light on the mechanisms of action of OFQ and dynorphin A on colonic motility. This may have important clinical implications since OFQ or its analogues may be beneficial for the treatment of refractory constipation in humans.